This invention pertains to reconfigurable magnetic workholding fixtures. More specifically, this invention relates to robust and easily re-configured fixtures comprising a flat magnetic chuck surface with magnetically attached modular supporting, locating and clamping elements that are useful for locating and securing workpieces, especially non-magnetic workpieces.
There is a need in manufacturing operations for durable and reconfigurable workholding fixtures especially for flexible manufacturing operations. The fixtures must be readily and accurately adaptable to hold different workpiece shapes for machining applications or the like. Sometimes the workpieces are similar or related part shapes such as cast aluminum cylinder heads for different engines. In other manufacturing situations the parts may be of unrelated design but requiring similar manufacturing operations. In these varied applications, the fixture reconfiguration or changeover from one part design to another has to be fast enough to meet the productivity requirements of current manufacturing systems.
Magnetic chucks have been available for holding some magnetic workpieces. For example, magnetic chucks providing flat holding surfaces are commercially available under the trade designation xe2x80x9cQuadsystemxe2x80x9d by Technomagnete S.P.A. These chucks are provided with a plurality of magnetic pole pieces often arranged in a rectangular grid on a base plate within peripheral walls. Apart from the coils that energize or de-energize the steel pole pieces, the chuck also includes high energy permanent magnets (neodymium-iron-boron magnets) interspersed among the poles. U.S. Pat. Nos. 4,847,582 and 4,956,625 appear to describe such flat surface chucks. Suitably placed and oriented electrical coils permit the chuck to be magnetized to different levels and demagnetized for locating, securing and removing a magnetizable work piece as is described in the specifications and drawing figures of these patents which are incorporated herein by reference. However, this chuck has not proven useful for securing and machining non-magnetic workpieces such as, e.g., cast aluminum alloys.
It is an object of this invention to provide a fixture utilizing a magnetic chuck for workpieces, especially non-magnetic workpieces. It is a more specific object of this invention to provide an easily reconfigured workpiece holding fixture comprising a magnetic base and modular workpiece supporting, locating and clamping elements.
In accordance with a preferred embodiment of this invention a workholding fixture basically comprises an electromagnetic chuck and several modular fixture elements for locating, clamping, and supporting a non-magnetic workpiece. Such a fixture is also useful for iron or steel workpieces.
A suitable magnetic chuck is one that can be energized and de-energized with an electric pulse from a stationary electric power source. Thus, the magnetic chuck provides a fast attach/release capability and a flat surface on which modular workpiece supporting, locating and clamping elements can be securely held by the magnetic force. Preferably, the strength of the field can be varied from a first level for sensitive and accurate modular element placement to a second, higher level for strongly securing the elements. In addition, the energized magnetic chuck,can maintain its magnetic attractive force even when the chuck is disconnected from the power source. This is important because it provides the required mobility of the workholding fixture to be transferred from one machining station to another machining station.
The autonomous integrated modular clamping or supporting elements are designed and constructed to be precisely located on the flat surface of the magnetic chuck and to then receive and securely hold a non-magnetic workpiece for a machining or other manufacturing operation. The work piece may be a nonferrous alloy such as an aluminum or magnesium alloy or the like. The workpiece is held spaced apart from the magnetic chuck by suitably located and cooperating supporting, locating and clamping members.
The supporting, locating and clamping elements may be individually carried on a suitable base plate. The base plate is preferably made of steel or other magnetizable alloy and sized so that the support or clamp is tightly held on the magnetized chuck plate. Support or clamp posts stand upright from the base plate. In one embodiment, the support is simply a post of predetermined length fixed at one end to the base plate with the other end configured to engage and support a surface of a workpiece. The length of the support post is determined by the shape of the work piece and its intended distance from the chuck surface. The clamp is typically a rotatable horizontal arm attached to the upper end of a post carried by a base plate. Clamp arms of varying shapes may be used to accommodate different workpieces.
In addition to the magnetically attached locating, supporting and clamping elements, the fixture preferably comprises a rigid guide rail and at least one locator element (which is also a support element) that are precisely located and mechanically fixed to the surface of the magnetic chuck. These mechanically fixed elements constrain the workpiece from transverse movement on the chuck surface. Movable locator elements used in combination with these fixed elements assure that the fixture is suitably reconfigured for different cylinder heads or other workpieces.
In its simplest terms the fixture comprises a magnetic chuck with a mechanically fixed guide rail and locator element, and at least one support element and one clamp arm element each secured to the surface of the chuck by magnetic force. Generally, a plurality of the magnetically attached elements will be located on the chuck surface to cooperate in holding the workpiece for a machining operation or the like. However, in a preferred embodiment of the invention, the height of the movable support elements and clamping elements are not fixed, they are adjustable to accommodate different workpieces.
In support elements or clamping elements of adjustable height, the posts can, for example, be fitted into a cylinder or other housing and threaded for screw height adjustment, or adjusted hydraulically. Such support and clamping elements are commercially available. Hydraulic height adjustment of post height may require that the base plate be modified to accommodate suitable hydraulic pressure supplying components.
The magnetic chuck and modular support and clamp elements allow a fixture to be easily assembled or modified. For example, support and clamp elements can be individually selected from a suitable inventory or storage magazine by a numerically controlled robot and precisely located on a magnetic chuck surface. The element-placing robot arm will have a suitable gripping adapter to grip the support or clamp element and place it base down on the chuck surface. Similarly, support and clamp elements are removed from the chuck when the need for the present fixture assembly is finished. The chuck is partially magnetized for element placement and removal and fully magnetized to secure the elements to the chuck in the working fixture.
The fixed guide rail and locator post(s) are located for general fixture applications. The magnetically attached locator, support and clamp elements are positioned for each workpiece configuration. They are positioned so that the clamp arms can press against a surface of the workpiece and force it against a suitable surface piece on the top of each support post. The workpiece is hoisted and placed against the support posts and the arms of the clamps rotated into position engaging the workpiece.
The assembled fixture can be situated horizontally or vertically, or in any desired attitude with the workpiece securely held by the clamping elements against the locator and support elements in a position spaced from the surface of the chuck. The combination of the mechanically fixed and magnetically fixed supporting, locating and clamping elements holds the workpiece for machining or the like, even with heavy stock removal.
In another embodiment of the invention the roles or positions of the large magnetic chuck plate surface and the support plates for the support or clamp elements are reversed. In this embodiment, the fixture system essentially consists of a large conventional, magnetizable steel fixture plate and a collection of modular fixture elements for locating, clamping, and supporting a workpiece. The modular elements are all mounted on magnetic holding bases of some common dimensions. These magnetic holding bases are smaller versions of the magnetic chuck plates described above. The magnetic holding base for the support or clamp elements can be an electropermanent magnet construction so that it can be activated or de-activated with an electric switch. Alternatively, it can be a simple lift magnet design activated by a mechanical lever. The magnetic holding base will carry a support and/or clamp element and optionally a hydraulic pressure supplying component. The role of the magnetic holding base is to provide a fast attach/release capability to the flat steel fixture plate and also a platform on which a modular fixture element can be affixed mechanically. In addition, the activated magnetic support or clamp base will maintain its magnetic attractive force when the base is disconnected from the electric source or as long as the mechanical lever stays in the same position. This portability provides the required mobility of the smart workholding fixture plate to be transferred from one machining station to another machining station.